A QSO sample that can be employed for cosmological measurements should comply with a series of criteria that are aimed at reducing possible contaminants/systematics that may prevent the determination of the intrinsic X-ray (corona) and UV (disc) luminosity and bias the final sample. Whilst the interested reader should refer to
[1]} for a complete description and implementation of the filters (see also [2]}, [3]}, [4]}) aimed at obtaining the final “best" sample for a cosmological analysis, a brief overview is provided below.
The main possible sources of contamination that may affect the flux measurements are dust reddening and host-galaxy contamination in the optical/UV and gas absorption in the X-rays. Moreover, any flux-limited sample is biased towards brighter sources at high redshifts and this should be more relevant to the X-rays, since the relative observed flux range is narrower than in the optical/UV.
Specifically, AGN with an average X-ray intensity close to the flux limit of the observation will be observed only in case of a positive fluctuation. This introduces a systematic, redshift-dependent bias towards high fluxes, known as Eddington bias, which has the effect to flatten the \(F_{\rm X}-F_{\rm UV}\)  relation.
Samples that include only quasars with a detection at X-ray energies might thus be affected such a bias. This is true even when variability is accounted for, in the case detections only are considered. One possibility is to include censored data in the analysis. Yet, the investigation of both the \(F_{\rm X}-F_{\rm UV}\)  and the distance modulus–redshift relations is far from trivial, since it strongly depends upon the weights assumed in the fitting algorithm. Therefore, one needs to find an alternative method to obtain an (almost) unbiased sample (see, e.g., Appendix A in [3]}).
Finally, QSOs with bright radio emission and/or broad absorption lines (BALs) should also be neglected. Synchrotron emission in a radio QSO can also include a bright X-ray component that adds to the one of the corona, whilst the strong absorption features observed in BALs, and usually attributed to winds/outflows, hamper a robust measurement of the QSO continuum in the UV.
